The present invention relates to a circulation type of sheet refeeding device, for example, a circulation type of document conveyance device in which sheets stacked on a stacker are fed to a platen glass one by one and exposed sheets are returned to the stacker so that the sheets can be repeatedly fed to the platen glass. More particularly, the present invention relates to an automatic circulation type of sheet refeeding device in which the uppermost sheet of a sheet stack is conveyed to the platen glass and returned to the stack after it has been exposed.
An automatic document feeding device (ADF) has already been provided which can effectively and automatically feed documents to a recording device such as an electrophotographic copier and a recording image reader. However, conventional automatic document feeding devices are applied to the copying operation in which only one side of a document is copied.
On the other hand, there is a great need of a copier which can copy images on a two-sided document onto one side or both sides of a recording paper, and also there is a need of an image reading apparatus which can read and record images on a two-sided document. Therefore, various reversible automatic document feeders (RADF) provided with document reversing function have been proposed until now. Those automatic document feeders (RADF) are disclosed in Japanese Patent Application Open to Public Inspection Nos. 216159/1984, 112738/1978 and 8834/1985, and Japanese Utility Model Open to Public Inspection No. 4548/1984.
On the other hand, a circulation type of document feeder (RDH) has been proposed in which documents stacked on a stacker are automatically fed one by one to a platen glass of a copier and the exposed document is returned to the stacker so that it can be exposed again.
When the aforementioned circulation type of document feeder (RDH) is operated by one circulation, a volume of copies can be made, so that the documents are circulated by the number of necessary volumes.
Recently, the applicant has proposed a new circulation type of document feeder composed of the aforementioned RDH to which a document reversing mechanism is added (Japanese Patent Application Open to Public Inspection No. 20228/1988).
When the aforementioned circulation type of document feeder is provided, a high speed continuous copying operation can be carried out and copied recording papers can be sorted in the order of pages. Further, when the copying apparatus is connected with a finisher which conducts a stapling and punching operation, all recording operations can be automatically carried out.
In the aforementioned circulation type of document feeder (RDH), documents are fed in the following manner: the front side of a document is set upward on the stacker; the lowermost document of the document stack is fed to the platen glass one by one; and the exposed document is returned on the uppermost document of the document stack. The aforementioned system will be referred to as a bottom-feed/top-restack system, hereinafter.
In the aforementioned circulation type of document feeder (RDH), that is, in the bottom-feed/top-restack system, the document surfaces are rubbed with each other, so that the document surfaces are stained and images are damaged. Especially when the document is written with a pencil, the aforementioned problems are remarkable.
In order to solve the aforementioned problems, a circulation type of automatic document feeder is disclosed in Japanese Patent Application Open to Public Inspection No. 37536/1981, which is operated in the following manner: the uppermost document of a stack is fed to the platen glass; and the exposed document is returned to the lowermost portion of the stack. In the aforementioned system, an exposed document sheet is inserted under a document stack when the stack is lifted by a periodic motion type of lifting means.
On the other hand, in the fields of an image recording apparatus such as an electrophotographic copier and a printing apparatus, various two-sided automatic recording apparatus are proposed, which can record not only on one side of a recording paper but also on both sides. In a conventional two-sided automatic recording apparatus, a document image is recorded on one side of a recording paper in an image processing section, and then the recording paper is temporarily stored. After that, the recording paper is conveyed again so that it is fed to the image forming section. The aforementioned type of automatic recording paper refeeding device (ADU) is disclosed in Japanese Patent Application Open to Public Inspection Nos. 82247/1984, 114227/1984 and 2241/1985.
In the aforementioned automatic two-sided recording copier, a recording paper, on one side of which an image has already been formed, is conveyed under the copying section and accommodated in a stacker section.
After a recording paper, on one side of which an image has already been formed, has been stacked in the stacker, it is refed under the condition that the trailing edge is taking the lead. Then, the recording paper is conveyed by a conveyance device through a conveyance guide, and enters the image forming section again. After that, a toner image formed on a photoreceptor is transferred onto the recording paper. Then, the transferred image is fixed by a fixing unit, and the recording paper is accommodated on a tray disposed outside the apparatus.
Even in the case of the aforementioned ADU, on a conventional stacker, recording papers on which images have been recorded, are stacked on the uppermost paper on the stacker, and the lowermost recording paper is fed one by one. Accordingly, the surface of the recording paper is stained due to the friction between the front and reverse sides of the recording paper. Therefore, it is preferable that the uppermost recording paper is fed and the recording paper on which the image has been formed, enters the lowermost portion of the stack. The aforementioned system will be referred to as a top-feed/bottom-restack system, hereinafter.
In RDH and ADU of the aforementioned top-feed/bottom-restack system, the following problems are caused.
(1) The structure of the lifting means which lifts the stack of papers periodically, becomes complicated.
(2) When the leading edge of the recording paper inserted under the stack, is curled, there is a high possibility that the leading edge interferes with the periodical motion type of lifting means.
(3) In the case where the sheet returning speed is high, there is a high possibility that the motion of the sheet conveyance device becomes unstable since its structure is complicated.
(4) Since the stack lifting means goes up and down each time a recording sheet is returned, noises are made.
(5) When the sheet size is changed, it is difficult to adjust the apparatus to the size.
The present invention has been achieved to solve the aforementioned problems of the prior art. It is a primary object of the present invention to provide a top-feed/bottom-insertion type of sheet refeeding device which can be stably operated even in a high speed operation and appropriately adjusted to various sheet sizes.